This study was conducted to evaluate the vertical bone height changes of the completely edentulous maxillary arch opposed by two different designs of implant mandibular overdentures: a) Two-implants supported overdentures. b) Four -implants supported overdentures. Materials and methods: Twelve patients were selected from the out-patient clinic, Faculty of Oral & Dental medicine, Cairo University.Patients were divided into two groups : Group (A): patiens had received two implant retained mandibular overdentures opposing conventional maxillary dentures. Group (B): patiens had received four implant retained mandibular overdenture opposing conventional maxillary dentures. Radiographic evaluation was carried-out to evaluate the vertical bone height changes using CBCT in the maxillary anterior and posterior regions. Results: The results of the present study revealed that in the anterior region of the maxillary arch, both designs of two and four implant supported overdentures (ISOD) showed an insignificant difference between the mean values of vertical bone height loss measured at the time of overdenture insertion, 6 months and 12 months after insertion. While significant difference in the percentage change of the vertical bone height loss was occurred between the two designs at the period of 6 -12 months after insertion .with increase in the 2 implant design than the 4 implant design. On the other hand , in the posterior region of the maxillary arch, the results showed that both designs of two and four implant supported overdentures (ISOD) showed an insignificant difference between the mean values of the vertical bone height loss measured at the time of insertion ,6 and 12 months after insertion . In comparing the two overdenture designs, there was a significant increase in the mean values of the vertical bone height loss with the highest mean for the two implant design (group A). Also the results of the percentage changes of vertical bone height loss for different treatment options showed that there is no statistically significant difference between the two treatment options at 6-months and 12 months follow-up periods on the other hand, for 6 -12 months period there was a significant difference for the 2 ISOD. Conclusion: Within the limitations of the present study, it may be concluded that there was no significant difference between the two treatment options as regard the maxillary posterior bone height loss, while there was significant increase in the percentage bone height loss in the anterior maxillary region in the two implant design. However in the four implant design, there was no significant difference between the anterior and posterior region in the four implant design